Field monitors are employed in airport Microwave Landing Systems to check the accuracy and quality of the signal radiated by the MLS antenna. For realism, the field monitor should measure the same guidance beam as would be measured by an airborne MLS receiver. However, such realism requires that the monitor antenna be located in the far field of the MLS antenna, far removed from the distortions encountered in the near field of the MLS antenna. It is generally recognized that far field antenna pattern measurements should be conducted at a minimum distance of 2 D.sup.2 /.lambda., where D is the aperture of the antenna and .lambda. is the wavelength of the radiated signal, if near field distortions are to be avoided. If this criterion is followed for a typical MLS antenna (D=12 feet, .lambda.=0.2 feet), the monitor antenna must be located at a minimum distance of 1440 feet from the MLS antenna.
Few airport environments provide an obstacle-free path of adequate length to permit antenna pattern measurements to be conducted in the far field. Moreover, the signals received by the monitor would likely be corrupted by refractions and reflections from airport installations and from aircraft movements on the runway. The preferred distance for location of the monitor antenna is therefore about 150 feet from the MLS antenna, ahead of such obstructions as runway approach lights. Clearly, the monitor antenna is then well into the near field of the MLS antenna and cannot measure the guidance beam as would a distant airborne receiver.
Near field distortion effects occur because the r.f. path delays between the monitor antenna and radiating elements of the array at the outer edges of the transmitting antenna aperture are significantly different from the path delays between the monitor antenna and radiating elements near the center of the transmitting antenna aperture. Near field effects may be overcome by applying certain compensating factors to the transmitting antenna to cause the guidance beam to be refocused at the monitor antenna. Such a procedure is obviously unacceptable as it results in distortion of the far field guidance beam.
An integral monitor antenna is known and has been used to monitor performance of the MLS transmitting antenna. The integral monitor antenna comprises a slotted waveguide or a similar array of antenna elements that extends completely across the transmitting antenna aperture in very close proximity thereto. Compensation for near field effects is built into the integral monitor antenna so that the signal output of the integral monitor antenna simulates the signal output of a monitor antenna located in the far field of the transmitting antenna. However, it is impractical to duplicate the structure of the integral monitor antenna, considering the necessary changes in scale, when the monitor antenna is to be located at a distance of about 150 feet from the transmitting antenna.
The present invention utilizes the Fourier relationship between the near field pattern and the aperture function to reconstruct the far field pattern of the MLS antenna from sample measurements taken by a monitor antenna located in the near field of the MLS antenna.
U.S. Pat. No. 4,926,186, issued May 15, 1990, to R. J. Kelly and E.F.C. LaBerge, the present inventors, for "FFT-Based Aperture Monitor for Scanning Phased Arrays", owned by the assignee of the present invention, discloses a method in which signal samples, take by an integral monitor antenna or a monitor antenna located in the far field, are processed by Fourier transforms to provide the aperture function of the MLS antenna. Obtaining the aperture function in such manner permits the identification of individual phase shifters or other components of the MLS antenna that may be faulty.
U.S. Pat. No. 4,553,145, issued Nov. 12, 1985, to G. E. Evans for "Method of Forming the Far-Field Beam Pattern of an Antenna", discloses a method based on Fourier transforms for obtaining the far field pattern of a rotating antenna from measurements taken in the near field. The method collects near field signal samples at critical points of the antenna pattern for a partial reconstruction of the far field pattern. Close synchronization of angle transducers at the transmitting antenna pedestal and at the monitor receiver is required.
Accordingly, it is an object of the present invention to provide a method for near field beam monitoring of a scanning phased array MLS antenna which provides a complete reconstruction of the far field beam pattern, and which can be accomplished during normal operation of the array.